In integrated semiconductor memories, digital information items are stored in a multiplicity of memory cells that are each connected to a bit line and to a word line. Volatile semiconductor memories, in particular DRAM (dynamic random access memory), have memory cells each having a selection transistor and a storage capacitor. The storage capacitor may be formed as a trench capacitor or as a stacked capacitor and the selection transistor may be formed in particular as a MOSFET (metal oxide semiconductor field effect transistor). One electrode of the storage capacitor is connected to one source/drain region of the selection transistor. The other source/drain region is connected to a bit line. The gate electrode is connected to the word line and at the same time forms a word line section. The gate electrode is isolated from a channel region of the selection transistor by a gate oxide. A multiplicity of selection transistors are connected to each word line.
Present-day semiconductor memories generally have segmented word lines each having a main word line (the master word line) and a multiplicity of word line segments connected to the main word line. The word line segments are usually formed from polysilicon and form the gate electrodes of the connected selection transistors. In general, the same number of selection transistors is connected to each word line segment of a word line. The metallic main word line has a greater electrical conductivity than the word line segments made of polysilicon. In order to be able to dimension the main word line such that it is wider for the sake of further improved conductivity, it is often connected to a plurality of rows of word line segments, for instance two rows of word line segments running one beside the other. As a result, it is possible to reduce the number of master word lines in relation to the number of word line segments.
The metallic main word line serves for rapidly transmitting the word line potentials to all connected selection transistors. On account of the increasing miniaturization of structure elements of modern semiconductor circuits and on account of the ever greater transmission speeds and clock frequencies, the word line segments are nowadays driven actively in order to achieve faster switching of the selection transistors connected to the word lines. In this case, in addition to a word line driver to which the main word line is connected, a driver segment is also provided for each word line element, which driver segment supplies the respective word line segment (and also in each case a word line segment of further adjacent word lines) directly with the respective word line potential. From the multiplicity of driver segments, the word line potential passes to all connected selection transistors more rapidly than if only the end of the master word line is connected to the word line potential. The potential provided for activating a word line is usually designated by Vpp and the potential provided for deactivating the word line is usually designated by Vnwll. If a segmented word line is driven actively, these two potentials are in each applied directly to all word line segments of the word line that is to be activated or to be deactivated.
On account of manufacturing tolerances, the electrical connections between the word line segments and the main word line may be at high impedance or completely interrupted, so that an individual word line segment cannot be brought, or cannot be brought rapidly enough, to the envisaged word line potential. Defective electrical connections may arise, for example, as a result of high-impedance contact hole fillings that connect a word line segment to the main line. In the case of a word line that is driven actively, there are even two contact hole fillings provided between a word line segment and the main word line. Within the driver segments of the word line driver, too, defective or high-impedance electrical connections such as contact hole fillings can prevent the word line segment that is to be driven from being supplied with the respective word line potential.
By way of example, it may happen that, on account of defective contact hole fillings within a driver segment, although the word line segment can be activated, it cannot be deactivated, or vice versa. Such circuit defects are based on production tolerances, particularly in the lithographic patterning of contact holes, if lateral positional errors occur for example during the production of etching masks, as a result of which conductive structures that are to be arranged one on top of the other are not connected to one another with sufficiently low impedance. Furthermore, surface alterations of produced conductive structures, contaminants or other influences may lead to electrical decoupling of individual word line segments.
Such word line segments, which are not supplied, or are not supplied rapidly enough, with the respective word line potential are influenced by electrical potentials in their vicinity after the end of the activation operation or deactivation operation and assume a fluctuating or “floating” potential, which is adapted to the respective ambient potential and is virtually unforeseeable. The selection transistors connected to these floating word line segments then switch in an uncontrollable manner, as a result of which the charges stored in the connected storage capacitors can pass onto the bit lines. This leads to read-out errors when reading from other, actually intact memory cells. Consequently, on the basis of a test pattern of tested memory cells, it is not possible reliably to localize which word line segments are floating.
Furthermore, in the case of segmented word lines that are driven segment by segment with the aid of driver segments, the localization of floating word lines is made more difficult when only one of the word line potentials Vpp and Vnwll to be provided is not fed, or is not fed rapidly enough, to the word line segment. This is caused by the design of the driver segments, which have an inverter comprising a pFET transistor and an nFET tranistor, of which one transistor is connected to the potential Vpp and the other transistor is connected to the potential Vnwll. The potential fed to the main word line turns on one of the two transistors, so that the connected word line segment ideally immediately assumes the envisaged potential. However, if one of the two transistors has no or only a high-impedance electrical connection to the word line segment, either only the activation or the deactivation of the word line segment functions.